The Magnolia State: A supporter of Democratic presidential candidate Barack Obama spoke outside Obama's Mississippi headquarters in Jackson, days before the state's primary.

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Washington — Barack Obama has cast himself as the candidate of hope, an idealist with no patience for mudslinging and division.

So as Hillary Rodham Clinton throws harder punches – a strategy that analysts say helped her win the Ohio and Texas primaries a week ago – how hard can he hit back without undercutting his message of uplift?

The question has come into high relief over the past week as Senator Obama – who is expected to win the Mississippi primary Tuesday but faces a stiff challenge next month in Pennsylvania – pushes back against a fusillade of criticism from the Clinton campaign.

Since his losses March 4 in Ohio, Texas, and Rhode Island, Obama and his aides have questioned Senator Clinton's claim that she has been "fully vetted," called for her to release her White House papers and latest tax returns, and suggested that her assertions of overseas policy experience were exaggerated.

"What exactly is this foreign experience that she's claiming?" Obama told reporters on his campaign plane last week, alluding to Clinton's "red phone" TV ad questioning Obama's readiness for an international crisis. "I know she talks about visiting 80 countries. It is not clear, was she negotiating treaties or agreements, or was she handling crises during this period of time? My sense is the answer's no."

A campaign surrogate, former New Jersey Sen. Bill Bradley, meanwhile, took aim at Bill Clinton's failure to disclose contributors to his presidential library. "Are there favors attached to $500,000 or $1 million contributions?" he said on "The NewsHour with Jim Lehrer" Wednesday. "And what do I mean by favors? I mean, pardons that are granted, investigations that are squelched, contracts that are awarded, regulations that are delayed."

Obama has promised voters a "different kind of politics," and before the results were in last Tuesday he insisted his campaign would not change course. "I have said consistently that we do things differently," he told reporters. "It's worked for us so far. And I'm not going to do things that I'm not comfortable in doing."

That night, however, his chief strategist, David Axelrod, told reporters that the campaign would not leave Clinton's salvos unanswered. "If she wants to make issues like ethics and disclosure and law firms and real estate deals and all that stuff issues, as I've said before, I don't know why they'd want to go there, but I guess that's where they'll take the race," Mr. Axelrod said. "If Senator Clinton wants to take the debate to various places, we'll join that debate."

That will require a tango, analysts say. Respond too harshly, they say, and he jeopardizes a chief part of his appeal. Don't respond at all, and the charges develop the patina of truth. Sen. John Kerry learned the latter lesson well four years ago with his slow response to the "Swift Boat" ads questioning his military service.

"Obama's got a real dilemma," says J. Michael Hogan, a professor at Pennsylvania State University in University Park who studies presidential campaign rhetoric. "If he doesn't respond to the criticisms and the attacks, then they go unanswered. But if he does, he betrays his own promise to change the tone of campaign discourse."

That may be partly what the Clinton campaign is aiming for with a biting series of attacks on his experience, candor, and character, says Dr. Hogan. "What they're trying to do is tempt him to abandon his high horse and say something that will reflect badly on him in terms of that commitment to be positive."

Clinton operatives wasted no time making just such a case. On Thursday, Clinton spokesman Howard Wolfson contended that with his fresh attacks on Clinton, Obama was "imitating Ken Starr," the independent counsel whose probe of the Whitewater real estate affair in the mid-1990s was denounced by Democrats as politically motivated.

The next day, the Clinton campaign convened a conference call with reporters to demand that Samantha Power, a Pulitzer Prize-winning author and unpaid foreign policy adviser to Obama, be fired after a Scottish newspaper quoted her calling Clinton a "monster."

"Senator Obama set a tone for this campaign and has talked about the politics of hope," Rep. Debbie Wasserman Schultz, a Florida Democrat and Clinton backer, told reporters on the call. "For either the candidate or his senior advisers to degenerate into negative personal attacks and name calling, which clearly was the result of frustration and anger over his losses Tuesday, is below the belt and out of bounds."

Ms. Power resigned later in the day. She said in a statement that her remarks were "at marked variance from my oft-stated admiration for Senator Clinton and from the spirit, tenor and purpose of the Obama campaign."

Tommy Vietor, an Obama spokesman, said in an interview that the campaign is highlighting differences between the candidates, not making personal attacks. "If the Clinton campaign feels like an honest discussion of her record is negative, we would disagree," he says.

Bill Carrick, a Democratic political strategist based in Los Angeles, contends that Obama is making a trade-off in dialing up his critique of Clinton. "For Obama, what's the worse option: maintaining an aura of his candidacy that has worked for him or being dragged down by the Clinton people and losing the primaries?" Mr. Carrick says. "If he's going to change the tone of politics in America, the first thing he's going to have to do is get elected."

The recent exchanges are a necessary if belated effort by the candidates to win over an electorate seemingly unable to settle on a front-runner, says Dr. Redlawsk. "One of the real challenges Democratic voters are having this time is that distinctions have not been drawn very clearly at all," he says. "We've had relatively little 'compare and contrast,' and I think in the end voters are left with less information than they would otherwise have.

"Now we're at the point," he adds, "where even the Obama campaign recognizes they have to make distinctions."